1. Field of the Invention
The invention relates to a system for capturing high contrast, detailed optical fingerprint images, and recording the fingerprint images as an object in a Fourier transform hologram and subsequently comparing a Fourier transform of high contrast, detailed optical fingerprint images presented in a coherent object beam with that recorded in a Fourier transform hologram fingerprint matched filter for verification purposes.
2. Description of the Prior Art
Typically fingerprint capture and verification systems include mechanisms for optically capturing images of a fingerprint, mechanisms for optically comparing or interrogating records of fingerprint images with contemporaneous fingerprint images, and signal comparison/computational processors (computers) for analyzing and providing output indicative or as a consequence of a match between a permanent fingerprint record and a contemporaneously captured fingerprint image. The preferred fingerprint record for such systems is a hologram or holographic interference pattern created utilizing an object beam and a reference beam emanating from a common coherent (laser) light/radiation source. The object beam has or contains information, i.e., a fingerprint image (or its Fourier transform). The reference beam contains no information or image. The respective beams interfere within a volume of a holographic recording medium generating an interference pattern. Holograms have the unique property of reconstructing the corresponding reference or object light beams when subsequently illuminated by a light beam corresponding with the recording object or reference beam respectively. [See generally Van Nostrand's Scientific Encyclopedia 8.sup.th Ed. "Hologram" pp. 1602-1604.]
When interrogating a hologram for pattern identification, comparison and/or verification purposes, a hologram of the Fourier transform of the image, variously described as a spatial or matched filter, is preferred. In an optical context, a Fourier transform of an image is basically performed by a lens transforming an image from spatial to phase domain at a plane containing the focal point of the particular lens. U.S. Pat. No. 3,716,301 Caulfield et al, [col. 4, 11. 12-64] presents an explanation generally describing both the nature of and how to record a hologram of a Fourier transform of a fingerprint image. [It should be recognized that the particular fingerprint image described by Caulfiel et al is dimensionally compressed vertically by a factor of 1/.sqroot.2.]
In the Applicant's co-pending applications Ser. No. 08/499,673 filed Jul. 7, 1995 and Ser. No. 08/694,671 filed Aug. 8, 1996 an optical device utilizing the phenomenon of total internal reflection and a holographic phase grating for capturing and provided a dimensionally undistorted optical image of a fingerprint is suitable both for: (i) creating a permanent record of the captured image and (ii) interrogating a previously recorded permanent finger print record.
It should be appreciated that while optically undistorted fingerprint images photographically recorded in transparencies or printed on photographic paper are suitable for manual or visual comparison purposes, because of thickness variations in the recording media, such transparency and print images typically should not be used directly for generating holograms of Fourier transforms of such images, i.e., the spatial or matched filters of the recorded fingerprint images.
Also, existing real time fingerprint verification systems which interrogate holographic Fourier transforms or spatial/matched filters of fingerprint images prerecorded onto an identification card are particularly prone to false positive verifications especially when the matched filters are flooded with light scattered into the interrogating optical beam by "wiping" smears or streaks on the fingerprint capture surface. In such instances, the Fourier transform of the scattered light in the interrogating beam partially correlates with the pattern recorded in the matched filter thus generating optical output at a detector indicating partial correlation. In particular, while it is recognized that the fingerprints of an individual are uniquely different from those of others, the degrees of general similarity between fingerprints of different individuals are typically greater than the degrees of difference. For example, spacing between print ridges and pores per unit length along a print ridge in the same region of right index finger human fingerprints can in fact essentially coincide except for one or two distinctive differences. It is also recognized that human fingerprints typically fall into distinctive patterns or groups, which fingerprint experts currently use for cataloguing purposes. The upshot is that an interrogating coherent object beam containing any fingerprint image when Fourier transformed, will generate a correlation light signal from any Fourier transform fingerprint matched filter. The intensity or magnitude of the correlation light signal is indicative of the degree of correlation between the real time Fourier transform interrogating object beam and the recording object beam used to create the particular fingerprint matched filter. [See U.S. Pat. No. 5,600,485, Iwaki et al, and U.S. Pat. Nos. 4,750,153; 4,837,843; 4,860,254 & 4,961,615, Wenchko et al which describe associative memory systems which utilize the described properties of holographic Fourier transforms matched filters in combination with Spatial Light Modulators (SLMs) for pattern recognition.] Thus scattered light flooding an input interrogating object beam of existing real time fingerprint verification systems can add sufficiently to the correlation optical light signal to change an actual negative correlation to a positive verification.
Existing real time fingerprint verification systems also present alignment and orientation problems. In other words, it is nearly impossible for a finger to be placed in exactly the same position twice on an input scanner surface. This means that it is nearly impossible to create an identical planar image of a fingerprint image (or its Fourier transform) that corresponds in position to that pre-recorded in a matched filter, a problem that becomes even more difficult when different scanner and processing optics are used respectively to capture and record the matched filter image and to capture and interrogate the recorded matched filter with an image.
Alignment typically refers to X-Y position correlation between the Fourier transformed image in the interrogating beam and that pre-recorded in the matched filter, i.e., assuming the Fourier transformed image in the interrogating object beam is in the same plane as the Fourier transformed image pre-recorded in the matched filter, it is the displacement of the interrogating image in the X-Y plane of the matched filter relative to the matched filter image. U.S. Pat. No. 5,541,994, Tomko et al [Col. 8 line 52-.Col. 9, 1. 38] addresses the problem of X-Y correspondence by scanning for location of output peaks of a Fourier transformed fingerprint image transmitted through a spatial light modulator (SLM) receiving input from stored reference filters to define an array of values.
Orientation typically refers to the angular or rotational position correlation of the interrogating image with the matched filter image assuming the respective images are at the same X-Y position, i.e., the degree to which the Fourier transformed interrogating object image is rotated relative to the Fourier transformed matched filter image. U.S. Pat. No. 3,716,301 Caulfield et al [Col. 7, 11. 28-66] suggests a both a dynamic and static optical solution to the problem of orientation. The dynamic solution suggestion involves rotating a dove prism to achieve "opto-mechanical rotational alignment" of the interrogating object image with that recorded in the matched filter [col. 5, 11. 55-62], The static solution suggestion involves spatially multiplexed Fourier transform fingerprint images recorded angularly around a common axis in the matched filter, the angle of incidence of the creating reference beam being slightly different at each different-rotational position. An affirmation-negation signal discrimination circuit receives input from detectors located at different positions to provide a threshold signal indicative of a matched. In contrast, U.S. Pat. No. 5,095,194 Barbanell [Col. 5, 11. 16-32] teaches a simpler dynamic solution contemplating movement of the finger on the input surface as the mechanism for achieving correlation of position of the interrogating Fourier transformed real time image with that recorded in the matched filter to produce a threshold optical signal at an appropriately located detector to detect a reconstructed reference beam pulse in the event of a correlation as the position of interrogating image sweeps around responsive to finger movements on the input surface.
Another factor affecting performance of coherent light--holographic matched filter optical systems for authenticating or verifying an identity is phase correlation. As previously mentioned, a coherent light fingerprint image taken from a transparency should not be used to create a holographic matched filter because of thickness variations in the transparency. Such thickness variations mean differences in optical path length, i.e., variations in phase in the plane of such Fourier transformed image recorded in a matched filter. Accordingly, when the matched filter is subsequently interrogated, real time, by a Fourier transformed of a captured fingerprint image, the magnitude of correlation will depend on the degree of correspondence (or lack thereof) of phase-of the interrogating object beams with that pre-recorded in the plane of the image recorded in the matched filter. Phase correlation problems also arises out of differences in optical path lengths between the recording optical systems creating matched filters and the optical systems comparing real time, captured images to those recorded in the matched filter. For example, even assuming alignment and rotational correlation of the respective interrogating and matched filter images in a credit card system of the type contemplated in U.S. Pat. No. 5,095,194 Barbanell, infinitesimal differences in position on the optical axis of the card matched filter in the verification apparatus can mean the difference between a threshold correlation signal or not.